1. Field of the Invention
The present invention relates to an optical scanning apparatus in which a photosensitive member is scanned with a plurality of laser beams modulated according to an image signal to form a latent image. In particular, the present invention relates to an optical scanning apparatus for controlling a sub scanning pitch of a plurality of semiconductor lasers, and an image forming apparatus using the optical scanning apparatus.
2. Related Background Art
In an optical scanning apparatus in which a laser beam emitted by a laser beam driving circuit of a semiconductor laser is modulated according to an image signal and a photosensitive drum is scanned with the laser beam by a scanner motor for raster scanning to form a latent image, it is necessary to provide a plurality of single beam semiconductor lasers or a plurality of multi-beam semiconductor lasers. In addition, in the case of a system composed of the multi-beam semiconductor lasers, it is necessary to detect scanning positions of the respective laser beams such that a pitch in the sub scanning direction becomes predetermined spaces.
On the other hand, there is a method in which a plurality of light receiving sensors are arranged on an axis substantially perpendicular to the scanning direction at predetermined spaces, and it is judged that the spaces between the laser beams become the predetermined spaces through the fact that each of a plurality of laser beams passes through a corresponding light receiving sensor (for example, JP 2000-180745 A, JP 10-090615 A, and JP 10-090616 A).
However, in the above-mentioned conventional example, high precision mounting is required for positioning the plurality of light receiving sensors. In particular, variations in the direction substantially perpendicular to the scanning direction may cause the cases where laser beams cannot be detected. In addition, time lengths or voltage values of signals outputted from the light receiving sensors are measured and scanning positions are calculated from differential values thereof. Accordingly, it is difficult to detect slight changes in the scanning positions between the plurality of laser beams.